Default enable the `ban_user_feature_flag` feature flag

What does this MR do and why?

Related to #330667 (closed)

Default the ban_user_feature_flag feature flag to enabled. This enables the ban/unban users functionality by default. Banned users are blocked, and their issues are hidden.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Follow the steps in https://docs.gitlab.com/ee/user/admin_area/moderate_users.html#ban-and-unban-users to ban/unban users

MR acceptance checklist

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